Design for Manufacturing in the Digital Age
Design for manufacturability (DFM) is a systematic approach to optimized manufacturing that focuses on the time and difficulty to complete steps within the manufacturing process.
DFM offers more than just savings on labor rates and machine overhead. By doing things properly the first time, you can save on rework costs, as well as help to ensure that by identifying problematic manufacturing issues early on, quality can be designed into products from the very beginning, and maintained throughout the development process.
Traditionally, DFM has been a fairly laborious process, consisting of checklists and tables. Nowadays, we can turn to software to do the tedious stuff on our behalf, and that in turn frees us up to do the proper engineering.
One such software is DFMPro, which is produced by Geometric, an India-based engineering solutions company. DFMPro integrates with popular engineering CAD packages, and promises a reduction of cycle time, high-quality products and lower product development costs for anyone wishing to give it a try in their company.
I downloaded the trial version of DFMPro for SOLIDWORKS 2016 to see what it was all about.
Start Me Up
Installation of the software is painless. The file is opened, a license manager appears, and you just point the browser to the location of the license file. When SOLIDWORKS is opened, a new tab is visible in the software’s main window. This is the interface where all the DFM stuff can be performed.
You can see in the image above that there are three options available as soon as you click the DFMPro tab:Start DFMPro, DFMPro Rule Manager and DFMPro Options (see Figure 1).
Let’s take a look at these in more detail. Start DFMPro is self-explanatory, and we will examine this in more detail later. First, let’s look at the DFMPro Rule Manager option.
DFMPro is fairly customizable in terms of requirements. And this customizability comes from the rules. When you run the DFM analysis, the software will analyze your CAD model according to what manufacturing process you select (e.g., milling or injection molding), and it will flag any errors according to a set of rules.
Say, for example, you want to design a part that is optimized for cost and which has the Standard Hole Sizes rule active. This is because an off-the-shelf drill bit is going to be a lot cheaper than a custom piece, and therefore, a nonstandard hole would require an expensive drill and would raise the cost per part.
You can click on the DFMPro Rule Manager and scroll down to the Standard Hole Sizes section, right-click, and press “Configure Rule” to see what is going on there (see Figure 2). You can add your own part sizes, and change the rules according to the requirements of your company. What is nonstandard to one company maybe commonplace to another!
Let’s take a look at a part as an example. DFMPro can be used for analysis of prismatic milling, turning, sheet metal work, injection molding, casting and die casting. For this example, let’s take a look at a part that we may wish to be manufactured with milling. The part in question is a rudimentary impellor (see Figure 3), similar to what you may find inside a turbocharger housing.
As this is just an example of manufacturability, we can ignore the traditional constraints that may dictate the manufacture of an impellor. The fins are definitely not optimized for fluid flow, for instance. And let’s just pick a material commonly used in aerospace for input into SOLIDWORKS—for this example, we will use 7075 aluminumalloy. It may not be the best material for a real-life impellor, but it is just an example. We won’t need to fire up our materials selector software.
So we load up the impellor SLDPRT into SOLIDWORKS; click on Start DFMPro; using the default rules file, we select Design For: Prismatic Mill; and press the Run button (see Figure 4).
After a couple of seconds, the analysis is complete and we see a couple of items flagged already. Apparently, the space shuttle grade 7075 aluminum alloy is not on the default budget materials list within DFMPro, so that has been flagged as being a cost enhancer. Also, there is a flag for “Fillets on Top Edges.” If you hover over the individual errors, a little box will appear that explains why the error exists (see Figure 5). Left-clicking on the error in the list will highlight the location of the error in blue within the main CAD window.
In this case, the rule that has failed is related to the fillets. The explanation reads as follows:
“For outside corners, chamfers are preferable over radii. An outside radius requires a form-relieved cutter and a precise setup, both of which are expensive.”
Ouch. That’s DFM for you. The tiniest detail can have costly ramifications. That’s why it pays to design these things out first. Let’s add a chamfer instead. We can add 7075 to the acceptable material database, or we can just click on Ignore. I will choose the latter (see Figure 6).
After adding a 1mm chamfer in place of the fillets, I run the analysis again, and the warning has disappeared. So that’s great. The part can be manufactured, and won’t incur too many extra significant costs as defined by the rules that are loaded.
We can take a look at some of those rules while we are on the subject. In addition to displaying the rules that have failed, the software displays the rules that have passed (which can be user defined, as mentioned).
Tool Accessibility is an interesting rule. If this item is flagged, it means that you are asking your CNC machine to do things that are probably impossible.
I add a basic extruded cut within the base of the CAD file, creating a small internal ledge. And onto that ledge, I create a small hole.
Figure 7 shows a cutaway view, with the internal ledge in blue and the arbitrarily dimensioned hole positioned in the face of that ledge. There is no way you could make that hole as it is shown here. You would have to drill completely from the bottom surface. The ledge would be problematic for a number of reasons, not least because it is a small feature and the tool would not be able to reach inside the ledge at such an angle. Running the analysis again yields the following:
Three out of 65 defined rules have failed, with two instances of tool accessibility violation occurring, one instance of standard hole size violation, and one instance of a flat-bottomed hole violation. Again, if you hover over each individual error, a box will appear that provides an explanation of the error and a recommendation:
In the case of the flat-bottomed hole error, the software recommends that I use a conical-bottomed hole for drilled holes and a flat hole for milled holes, and because my arbitrary hole diameter of 0.662mm is a bit nonstandard, it recommends that I opt for a 0.65 or 0.7mm hole. But this particular hole is flagged as being unreachable anyway, so that doesn’t matter. The solution is clear. Remove this hole, or else add another cut to improve accessibility.
Generally speaking, the rules for milling correspond to extra machining steps. The step may involve a machine change or a tool change that results from overly complex geometry or poorly thought-out geometry. Either of these changes will increase costs via adding machine time, or even additional work hours.
After your analysis is complete, you can generate a report by clicking the Generate Report button at the bottom of the DFMPro pane so that your findings can be shared with colleagues (see Figure 8).
As mentioned, DFMPro can also perform analysis of other manufacturing processes.
The injection molding analysis, for example, can detect potential design errors that can result in uneven cooling, warpage, sinkmarks, etc., and will make recommendations based on well-established injection molding rules to help ensure a successful design.
The video below provides an introduction to some of the DFMPro basics across the different processes:
DFMPro promises to capture bad design decisions before they become bad manufacturing decisions.
What’s the difference between a bad design decision and a bad manufacturing decision? Ask Samsung how much money it lost off its stock value in a single day when it had to recall the Galaxy Note 7.
$18 billion bucks is a considerable figure. And that is kind of the point of DFx and all of its children (DFM, design for assembly [DFA], etc.): Identify issues at a point in the design phase where it is less costly to implement the design changes and while it is cheap enough to implement them. Deleting an ill-considered CAD file in the design phase costs nothing but the cost of electricity, work hours and software licensing.
Scrapping a production-level mold after it has been manufactured because it is leaving sinkmarks on your product is a much more serious and costly error to correct.
If you would like to see how DFMPro can help you or your company to minimize costs, or if you’d just like to know more about DFM in general, then you can pop on over to download a trial copy of the software from the product’s website.
And if you are interested in this subject in more general or academic terms, then I recommend that you get yourself a copy of Design for Manufacturing handbook by James Bralla or Design for Manufacturing: A Structured Approach. These books are good to begin with DFM /DFMA concepts.